Shadow ball mewtwo not mewtwo in general, other sets are pretty niche at best and are outclassed in everything but psychic.
Im pretty sure they're setting up for raid days of the legendaries. I think everyone would spend crap loads of money on the game if the came out with roar of time dialga in like 2 years that out DPS's rayquaza and having that massive TDO. They did the same thing to metagross in gen 3, giving it trash moves and its getting a CD in a few days, they are planning for down the road.
Sidenote, palkia's 3rd best dragon type, behind salamence by only .03 dps with a lot more tdo. The new pokemon arent trash. Yes they arent as good as other options, but far from trash